BMC12H-installation-manual.pdf - Servo2Go

Appendix A: Motor and Drive Selections
Determine voltage and current requirements for the motor, based upon maximum velocity
and torque.
Generally motor voltage is proportional to the motor speed and motor current is
proportional to motor shaft torque. This relationship is described by the following
equations:
V b = Ke * ω m
V t = (I m * R m ) + V b
T = Kt * I m
where:
V b = Back EMF voltage
Ke = Motor Voltage Constant
ω m = Motor Speed
V t = Terminal voltage (DC power voltage)
I m = Motor current
R m = Motor terminal resistance (winding)
T = Motor load torque
Kt = Motor Torque Constant
(volts)
(V/KRPM)
(RPM)
(volts)
(amps)
(ohms)
(lb-in or Nm)
(lb-in/Amp or Nm/Amp)
Note: Usually the motor manufacture data sheet will contain the Kt and Ke values.
Determining the motor maximum speed and torque will determine maximum voltage and
current.
V max = (Ke * ω max ) * 1.10
For example, a motor with a Ke of 10 V/KRPM and required speed of 3000 RPM would
require 33 Vdc minimum to operate successively (the IR term was neglected in this
calculation).
It is recommended to select a power supply voltage with a 10 % higher than the maximum
required for the motor used in the application. This is a safety percentage to account for
the variances in the motor Ke, Kt and losses in the system external to the drive.
Maximum current required for your application and from your system (drive/motor) can be
calculated as follow:
I max = T max / (Kt * .95)
For example, a motor with Kt = 1 lb-in/Amp and a maximum torque required of 4 lb-in
would require a minimum of 4.2 Amps. The term (Kt * .95) is called the effective Kt
(effective torque constant) of the motor which assumes a hot value for the application.
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MCG Inc. BMC 12H – Hardware Installation